The present invention relates to a control system for steam flowrate and steam pressure in the interconnecting line between a high-pressure and a medium-pressure steam line, said interconnecting line being a bypass line in a steam system that incorporates steam turbines. Control systems of this kind are widely used as constituent parts of steam systems in the chemical industries and comprise substantially a combination quick-opening and control valve, a flowmeter with root-extracting and multiplier relays installed on the feed-stream line to the steam turbines, and a pressure controller installed on the line of the lower pressure rating.
Control systems for steam flowrate and steam pressure are intended to serve as a plant safety element in integrated industrial chemical plants to prevent damage in the event of a sudden failure of steam consumers, such as steam turbines or chemical processes.
Production processes in industrial chemical plants, such as ammonia and ethylene plants, release process heat at high temperature levels while steam of a certain definite pressure level is needed as auxiliary fluid for various chemical reactions. In addition, these plants require motive power for compressors, pumps or alternators. Because of these conditions and requirements, it is reasonable to utilize the process heat for steam-raising at high pressure, to expand the high-pressure steam in back pressure steam turbines for reducing its pressure to the process steam pressure level while obtaining motive power for compressors, pumps or alternators, and then to admit the process steam as auxiliary fluid from the medium pressure steam system to the process stages where steam is needed.
Plants designed along this concept are generally referred to as integrated plants. They usually comprise a high pressure turbine whose exhaust steam is sent to the medium pressure steam ling. The characteristics of the product obtained from chemical reactions by admixing steam as auxiliary fluid depend on the steam flowrate not only with regard to product quantity but particularly with regard to product composition, i.e. on a constant flow ratio of steam to starting material. A constant pressure level is required for process economy because a major part of the processes applied today is performed at an elevated pressure of 20 to 80 atm. g., for example, and the process yield is frequently conditional upon the pressure, i.e. the rate of conversion decreases as the pressure drops. In addition, the process steam admitted to the catalyst-filled reforming tubes exposed to external firing for the production of synthesis gas is expected to maintain a constant temperature level, i.e. in the event of a failure of the steam supply and exposure of the tubes to radiation heat, the outside wall temperature of the reforming tubes would be subject to overheating with consequent damage to the catalyst.
It is known that steam turbines are equipped with an emergency trip on the steam line to safeguard immediate interruption of the steam supply in the event of a failure. Any response of the emergency trip entails necessarily an interruption of the steam supply to the medium pressure steam line. As the steam consumers connected to this line are not at the same time shut down immediately, said consumers being either further steam turbines or process stages, the steam pressure is bound to break down at these points with consequent shutdown of the entire plant as a result of the response of the process steam ratio controller.
According to German Patent OS 1,927,509, it is known to prevent an excessive pressure drop in the medium pressure line with the aid of a steam pressure reducing valve with a quick opening device on a bypass line to the feedline of a steam turbine, i.e. between the high pressure line and the medium pressure line. The known steam pressure reducing valve with a quick opening device appears to satisfy the requirements with regard to change-over times and maintaining steam flowrates and steam pressure levels in a prime mover facility comprising steam turbines of high pressure, medium pressure, and low pressure ratings. Any transient reduction of the steam flowrate in medium pressure and low pressure steam turbines will entail solely a reduction of the output, but will not cause the materials of construction of the engine to be exposed to higher stresses. The known devices are, however, not adequate to meet the requirements that must be satisfied by a control system intended to ensure the change-over in an industrial chemical plant comprising an integrated steam system. The steam from the medium pressure system of such a plant is used, for example, in further steam turbines and, among other purposes, serves as process steam in fired tubular reactors for the cracking of hydrocarbons. Referring, for example, to an ammonia synthesis plant for the production of 1,000 tons/day of NH.sub.3, the medium pressure system of the plant, including superheater, has a volume of approximately 10 m.sup.3 and, therefore, contains approximately 100 kg of steam at a specific steam volume of 0.1 m.sup.3 /kg. As steamrequirements of consumers connected to the medium pressure system of such a plant amount to approximately 150 tons/hr. or approximately 42 kg/sec., the steam pressure will drop within as short a time as one second from the moment of steam supply failure to approximately 50% of its setpoint value. This significant pressure drop and steam shortage will cause an emergency shutdown of the entire plant and overheating of the reforming tubes exposed to external firing.